Driving the Golden Dragon LED
e
Application Note
INTRODUCTION
The Golden Dragon LED is OSRAM Opto
Semiconductors’ high performance LED
requiring special considerations in thermal
management and electrical implementation.
This application note is intended to help the
design engineer with the special electrical
considerations of the Golden Dragon LED.
With a higher current there is higher power,
and therefore more heat to dissipate. The
Golden Dragon LED package is optimized
for removing this heat efficiently. With an
integrated heat slug (also known as a heat
spreader) the thermal performance is far
superior to standard LEDs.
Golden Dragon LEDs are delivered on tape
and reel. It has a flat top to allow pick-andplace machinery installation. All contacts
(including the heat slug) are soldered in
place using standard infrared reflow
processes (Surface-mount component
processing). Up to now the Golden Dragon
is the only high power LED in the market
which is capable to be processed according
to these cost effective standard assembly
techniques. ESD handling guidelines should
be followed when handling the Golden
Dragon LED.
Basic structure
Figure 1 shows the internal structure of the
Golden Dragon LED.
Leads
Dielectric
Figure 1 Structure of the Golden Dragon
LED
There are large leads for a strong mechanical attachment to the printed circuit board
(PCB), and to assist proper orientation of the
part during reflow soldering.
The semiconductor die is directly attached to
the heat slug. This heat slug is cast inside
the molding compound, which forms a
reflector cup around the die. The heat slug is
exposed on the bottom of the part for IR
reflow soldering to the PCB to provide a very
low thermal resistance from the die to the
PCB it is mounted to. The die is covered
with an optically transparent encapsulation
material to protect the die from the ambient
environment.
Bond Wir
Die Attach
Heat Slug
Die
Solder
Molding Compound
Solder Pads
Aluminum Plate
February 2, 2005 page 1 of 14
DESIGN CONSIDERATIONS
Thermal design
Because the Golden Dragon LED has a high
power rating, special consideration must be
made to optimize the thermal performance
of the entire system.
OSRAM Opto Semiconductors has released
an application note specifically addressing
thermal design for the Golden Dragon LED.
(The application note is titled “Thermal
Management of the Golden Dragon”). For
more details, please consult that application
note.
Optical design
The scope of this application note does not
include details of optical design. However it
is an important step in the lighting system
design and should not be ignored. Optical
design must target the highest efficiency to
reduce the LED light output requirements
and therefore the driver and heat-sink
requirements.
Electrical design
Semiconductor technology differences
There are two technologies used to produce
LEDs: InGaN and InGaAlP.
Different colors can be achieved with these
two technologies. InGaAlP is used to
produce colors from Green (570nm) to
Super Red (632nm). It has a forward voltage
around 1.8V to 2.3V, depending on the
color. InGaN is used to produce colors from
Blue (460nm) to True Green (528nm) and
phosphor based colors like White (typ.
3250K or typ. 5600K). It has a higher
forward voltage around 3.2V to 3.8V,
depending on the color. Be sure to check
the data sheet for the specific LED you are
using to get the correct information.
High current
The Golden Dragon LED is a high current
LED capable of operation at current levels in
the hundreds of milliamps. InGaAlP products
(Amber-Red and Yellow) can operate from
100mA up to 750mA. InGaN products (Blue,
Verde Green, True Green, and White) can
operate up to 500mA.
This high current develops a great deal of
power to dissipate in the LED. This power
can be up to two Watts in specific products.
OSRAM Opto Semiconductors will continually develop improvements to the Golden
Dragon LED. Please check the data sheets
for the latest performance data.
Steep If vs. Vf slope
The Forward Current vs. Forward Voltage
curve of the Golden Dragon LED is very
similar to any other LED. It has a steeper
slope of the I
area. This makes for rapid changes in
forward current with small changes in
forward voltage. The graph in Figure 2
shows this characteristic. Increasing the
current in the diode will not increase the
forward voltage by a significant amount.
0.5
0.4
0.3
0.2
Forward current (Amps)
0.1
0.0
0.0 0.4 0.8 1.2 1.6 2.0 2.4 2.8 3.2
Figure 2: Graph of Forward Voltage versus
Forward Current for a typical yellow Golden
Dragon LED.
vs. Vf curve in the high current
f
Forward Voltage (Volts)
February 2, 2005 page 2 of 14
Note that a 0.1 Volt change in forward
voltage is marked, and the indicated forward
current changes approximately 100mA. This
is approximately a 40% change in current
with a 5% change in forward voltage.
The intensity of the Golden Dragon LED is
closely linked to the forward current. With a
40% change in current, the intensity will
change approximately 40%. To properly
control the LED intensity, current control or
current limiting is mandatory.
Temperature coefficient of forward
voltage
All LEDs exhibit a change in forward voltage
as the junction temperature changes. This
temperature coefficient of forward voltage is
published in each data sheet of individual
LEDs. InGaAlP LEDs (Yellow and Amber
Red) have a coefficient of between -3.0mV/K
to -5.2mV/K, and the InGaN LEDs (Blue,
Verde Green, and White) have a coefficient
of between –3.6mV/K and -5.2mV/K. Check
the data sheet for the specific part you are
using to find this coefficient for your designs.
Intensity changes over temperature
variations
All LEDs also exhibit a change in intensity as
the junction temperature changes. This is a
result of changing efficiencies in the
semiconductor, and not a result of the
change in the forward voltage over temperature changes. This temperature change is
non-linear, but is represented in graph form
in all data sheets. Check the data sheet for
the particular LED you are using for this
graph.
Example of critical data sheet
information
The published Forward Voltage of the thin
film amber-red Golden Dragon LED (LA
W5SF) is provided as a minimum (2.05V), a
typical (2.4V) and a maximum (2.65V). This
is the range that the LEDs can be delivered
from production. This voltage is tested at a
specific current. It is best to use any LED as
close to the test current as possible. It is
important to verify operation over this
voltage range to be sure operation is in the
safe range.
The published thermal coefficient of forward
voltage for the thin film amber-red Golden
Dragon LED is -5.2mV/K. The published
maximum junction temperature of the Thin
film amber-red Golden Dragon LED is
125°C. It is important to verify operation over
the specified operating temperature range to
assure that the maximum junction temperature is not exceeded.
The published maximum current of the thin
film amber-red Golden Dragon is 750mA.
All conditions (input supply variations,
temperature variations, and production
variations) must be evaluated to assure the
maximum current is not exceeded.
HOW TO DRIVE A HIGH CURRENT LED
LIKE THE
LED circuit arrangements
Due to the high slope of the Forward Current
vs. Forward Voltage graph (Figure 2) it is
strongly recommended to only connect the
Golden Dragon LED in a series arrangement
with some current control for each series
string in the system. As described in the
application note titled “Comparison of LED
Circuits”, a matrix circuit has uncertainties in
the LED current that result from a mismatch
of the LED forward voltages. The Golden
Dragon LED will have this behavior but more
so.
Series resistor current limiting
Standard LEDs, like the Power TOPLED®,
typically employ a series resistor to limit the
forward current. With 350mA through a
series resistor, and a 12V supply, the
resistor power dissipation can easily exceed
3 Watts when used with a single Golden
Dragon LED.
GOLDEN DRAGON LED
February 2, 2005 page 3 of 14
Putting more LEDs in the string, and thus
A
−
=
reducing the resistor value, will reduce the
power dissipation in the series resistor. With
the small resistances resulting, the supply
voltage variations will cause larger current
variations in the LEDs. Figure 3 shows the
different effect on the current with supply
voltage variations. (A typical automotive
lighting application will see a variation from
9V to 16V)
400
350
)
300
250
200
LED current (m
150
100
50
9
10
11 12
Supply voltage (V)
1 Dragon
3 Dragons
4 Dragons
13
(37.5
(27.0
(18.0
14
)Ω
)Ω
)Ω
15 16
Figure 3: Comparison of LED current
variations with supply voltage variations
The smaller resistor creates a larger current
variation in the LEDs from the minimum to
the maximum supply voltage. This variation
in current will create a variation in light
output of the LED. There is a possibility that
the maximum forward current (as published
in the data sheet) will be exceeded when the
supply is at its maximum. To minimize this
variation, maximize the resistance by
reducing the number of LEDs in each string.
With high power LEDs, the resistor is kept at
a minimum to minimize power dissipation.
These are mutually exclusive requirements;
therefore a balance must be achieved with a
compromise. High power resistors can be
expensive, and assembly of a high power
resistor can add significant cost. (i.e. hand
soldering, selective soldering, clinching, antivibration mounting.) These factors must also
be considered when determining the
balance.
Example series resistor calculations
There are many factors that affect current in
the LED during operation:
Supply variation
First, the supply voltage has some variation.
(Typically 5% to 10%, automotive experiences a variation from 9V to 16V with
nominal being in the 12.5V to 13.5V range.)
As we discussed previously, the supply
variation can add a significant current
variation in the LEDs.
For example, let’s start with a low cost 5%
regulator supplying 12V (V
reduce the large voltage swings typical in an
automotive lighting application. If we put
three LEDs in a string, each with 2.4V (V
typical for a thin film amber-red golden
dragon LED), the series resistor will have a
4.8V (V
) drop at 0.350A (I
resistor
results in a 13.7Ohm resistor dissipating
1.68W (P
resistor
).
VnVV
*
diodesupplyresistor
resistor
R
R
resistor
V
resistor
=
I
diode
V
8.4
A
350.0
IVP
=
*
=−=
7.13
Ω==
dioderesistorresistor
==
At the limits of the regulator tolerance, the
supply voltage increases only 0.6V (V
12.6V maximum). The voltage dropped
across the resistor increases to 5.4V, and
the current increases by 0.044A. The LED
now passes 394mA.
). This would
supply
diode
VVVV
8.44.2*312
WattsAVP
68.1350.0*8.4
). This
supply
=
f
February 2, 2005 page 4 of 14
VnVV
*
resistor
diode
−=
VI
=
resistordiode
diodesupplyresistor
VVVV
4.54.2*36.12
=−=
Resistance/
AVI
394.07.13/4.5
=Ω=
Temperature Variation
The second factor affecting LED current is
the temperature coefficient of the forward
voltage of the LED. The data sheet for every
LED gives a coefficient for the forward
voltage with respect to the junction
temperature. At higher temperatures, the
forward voltage of the LED will decrease.
For the InGaAlP thin film amber-red LED,
the coefficient is –5.2mV/K.
KT
60
=∆
VKKV
3.060/0052.0
−=×−
VLEDsV
9.033.0
−=×−
With a temperature rise of 60K above room
temperature, the forward voltage of each
LED drops 0.3V. With three LEDs in a string,
(in an attempt to reduce power dissipation in
the series resistor) the forward voltage
across the string will drop 0.9V as a result of
the temperature change.
The effects of supply variation and temperature variation add. with a 5% tolerance on a
12V supply, and a 60K temperature
increase, there is a possible total variation of
1.5V across the series resistor. This
increases the current in the LEDs by a total
of 0.11A. The LED now is passing 0.46A.
Production variation
The third factor affecting LED current is
production variation of its forward voltage.
The data sheet of the Thin film amber-red
Golden Dragon LED gives a room
temperature forward voltage variation of
0.6V. With a design targeting the nominal
value, this can be seen as a ±0.3V
tolerance. This voltage change adds with the
first two effects creating a possible total
variation of 1.84V across the series resistor
in this application.
0.3V----Production
0.9V----Temperature
0.6V----Supply
0.3 + 0.9V + 0.6V = 1.8V
So, the voltage across the resistor can
increase by 1.8V. The current in the LED is
now 0.48A. This is not yet at the maximum
current published for the thin film amber-red
Golden Dragon LED, but heat dissipation at
this current level may cause the maximum
junction temperature to be exceeded. This is
still significantly above the design intent of
the LED. The design must account for this
much variation to prevent LED damage. The
power dissipated in the LED and resistor will
increase substantially, and must be taken
into consideration. The current could also
decrease when these tolerances move in the
opposite direction. If all the tolerances were
in the opposite direction, the LED current
would drop to 0.2A. This could create
problems from intensity variation and the
specification may not be satisfied.
Special consideration must be given to these
factors to be sure the LED’s maximum
current rating and the maximum junction
temperature are not exceeded at any time in
the application when using a series resistor.
This means the LED must be used at a
nominal level far below its capacity. Using
the Golden Dragon LED at a reduced
capacity with a series resistor will require
more LEDs. This can significantly increase
system costs. In most applications, the cost
saved by using only the needed Golden
Dragon LEDs and eliminating the special
assembly costs of a high power resistor, will
easily cover the cost of a current control
supply.
February 2, 2005 page 5 of 14
CURRENT CONTROL
The current control supply can often be
assembled on the same board as the LEDs
to save on assembly costs. When doing this,
ensure the current control supply does not
excessively heat the LEDs.
In this example, the voltage regulator used
could have been configured as a current
source with no added cost to the system. If
the regulator had been eliminated from the
example, then the wide variations typical for
an automotive application (9V to 16V) would
have overstressed the LED more than what
we saw in the example.
Designing in a current control supply has
several benefits:
• Reduced LED count and thus lower
costs
• The current control supply can be all
SMT components, and a high power
resistor can be eliminated. This
significantly reduces assembly
complexity. Again this will give a cost
savings.
• Reduced power dissipation at the
higher temperature, higher supply
voltage and lower LED forward
voltage conditions (as compared to a
resistor drive).
• Constant intensity output will give a
better, more uniform appearance,
and can better satisfy a tight
tolerance specification.
Current control methods
There are two technologies to consider
when designing a current control driver for
the Golden Dragon LED. The first is a linear
current control driver, and the second is a
switch mode current control driver. This
application note will consider the basics of
using both, but will not detail designs.
Please seek information from the individual
IC manufacturers for designing with their
parts.
Linear current supply
Very similar to a voltage regulator, the linear
current supply uses a linear pass element
with a feedback mechanism that regulates
the current in a path rather than the voltage
at a node (see Figure 4).
Vsupply
Bias Control
Feedback
Figure 4: Principle of a linear current control
Many adjustable voltage regulators can be
configured to operate as a current regulator.
(The IC manufacturer will have suggestions
on maintaining regulator stability in these
configurations) There are specialty parts
designed as a current source or sink for
other applications that can be enlisted to
drive an LED. (battery chargers, solenoid
drivers, etc.)
The linear current supply can only be used
when the input voltage is always higher than
the output voltage. Headroom must also be
added to account for drop voltages in the
driver circuits. Otherwise there is not enough
voltage available to operate the LEDs.
The advantages of a linear current supply
are its simplicity of design, and low
component cost. A linear current supply has
a disadvantage in that in regulating current,
a large amount of power must be dissipated
by the supply. This can occur when there is
a large voltage drop across the supply. (This
dissipation will always be lower than a
resistor drive where the current will increase
as the voltage increases.) This often
requires a heat sink. Since the Golden
Dragon LED requires a heat sink in most
applications, the two can be combined into a
single heat sink.
February 2, 2005 page 6 of 14
Linear current control supply design
example
Let’s consider an application example
requiring three Golden Dragon LEDs in a
string. (Three LEDs are needed based on
minimum light requirements) We will use an
input supply voltage of 15V (V
With 3 LEDs (n) each having a 2.2V forward
voltage (V
), the current control supply (or
f
driver) is left to drop the remaining voltage
(V
).
driver
VnVV
*
driver
driver
−=
=
*
diodesupplydriver
VVVV
4.82.2*315
=−=
IVP
diodedriverdriver
94.2350.0*4.8
==
The driver needs to drop 8.4V, so the power
dissipated is 2.94W, which can be very
difficult to dissipate depending on the
maximum ambient temperature.
To reduce this dissipation, add an LED to
the string so the power is used by the
additional LED to generate light, not being
wasted in the supply. The power will still
need to be dissipated by the additional LED,
but more light is generated for use.
This additional light reduces the required
current. This means the Dragons only need
to run at about 280mA, and there is still a
surplus of light. By adding one LED to the
string, the supply now has to drop 6.2V. In
addition, with the LEDs operating at 280mA,
the power to dissipate in the current control
supply is 1.74W. This is manageable with
only PCB copper area around the driver IC,
or a small heat sink depending on the
maximum ambient temperature of the
system.
From this example we can see that adding a
single LED to the string can make the whole
system more thermally manageable. This
improves the overall system thermal management by reducing power loss in the
driver and increasing margin of the design.
This will improve the reliability of the entire
lighting system. In every case, the designer
WattsAVP
Supply
).
must evaluate all three parts of the design:
thermal system performance, electrical
system performance, and system assembly
efficiency, to properly optimize the system.
Examples of Linear current control
supply circuits:
Figure 5: TLE4242G, an example of a linear
current control
Figure 5 is an example of a linear current
control, the TLE 4242G from Infineon
Technologies. This circuit offers simplicity,
and is suitable for use in automotive
applications. There is a PWM input to control
LED brightness, or with a constant low, the
part will shut down and consume less than
1µA. There is also a status feedback for
diagnostics. The part includes overtemperature and short circuit protection.
Figure 6: LM2941, an example of a linear
current control
February 2, 2005 page 7 of 14
Figure 6 is an example of a linear current
control, the LM2941 from National
Semiconductor. It offers simplicity in design
and application.
Ω
0.5
10K
LF442
-
+
National LM20
Vout
Vin
GND
NC
91K
169K
2.87K
6.98K
499K
LF442
-
+
10K
Figure 7 LM2941, an example of a linear current control
Figure 7 is another example of a linear
current supply using the LM2941 from
National Semiconductor. It offers a
temperature compensation of the current
level. The current is reduced at higher
temperatures to allow use of a smaller heat
sink while not violating the maximum
junction temperature of the LED at higher
operating temperatures. Both circuits are
suitable for automotive applications and offer
over-temperature protection.
Switching current supply
The switching power supply is well known in
the mobile appliance marketplace. Typically
used to conserve battery power, the
switching power supply is desired for its
efficiency, which is its primary advantage.
This efficiency allows a switching power
supply to control the voltage for a large
current with little power dissipation. This also
applies to controlling the current in a load
with very little power dissipation. A switching
current control supply has a secondary
advantage that it often does not need a heat
when large numbers of LEDs are used in a
single application.
A switching current control supply is a
frequency-based device. This adds to the
complexity of the design. It must be carefully
designed to be frequency stable, and not
radiate electromagnetic noise in undesirable
spectrums. The latter is known as designing
for electromagnetic compatibility (EMC).
Individual manufacturers of switching power
supply integrated circuits can provide
assistance in both of these requirements.
Compared to a linear current control supply,
the switching current control supply can
have a higher cost and more components.
This cost disadvantage can often be offset
by not needing a heat sink and redundant
linear current control drivers, but this must
always be evaluated for each application.
sink to keep cool. This becomes valuable
National L 2941M
Adjust
On/Off
GND
10K
INPU T
10 Fµ
OUTPUT
100 Fµ
9V to 16 V
On/Off
February 2, 2005 page 8 of 14
Different topologies of switching power
supply current control
There are three primary topologies of
switching power supplies:
The buck regulator
The boost regulator
The buck-boost regulator
Determining which topology to use in the
design is based on the input and output
voltage levels:
The buck regulator can only function when
the supply voltage is always higher than the
load supply plus some voltage drop from the
buck circuitry.
The boost regulator can only function when
the supply voltage is always lower than the
output voltage minus some voltage drops
from the boost circuitry.
The buck-boost regulator can function when
the supply voltage can vary above or below
the load voltage, or when the load voltage
can vary above and below the supply
voltage. The former can occur when multiple
sources are used, or a source varies widely.
The later can occur with a current control
supply.
The buck topology is used most often when
the power dissipated in a linear current
control supply would be excessive.
The boost topology should be used when
there are too many LEDs to drive using a
linear current control supply. As the number
of LEDs increases, and the cost (and heat)
of the linear current control supplies also
increases, a cross-over point is quickly
found where the switching current control
supply is significantly cheaper than the linear
current control supplies. The LEDs are
arranged all in one string, which makes the
output voltage higher than the input supply
voltage. Putting more LEDs in a string
reduces the number of strings in the system,
and therefore reduces the total number of
current control drivers needed.
The buck-boost topology is often used in
applications where the application is
powered from the AC mains and the rectified
voltage varies from 0V to the peak voltage.
This topology is also used when the
application must operate from many varying
supplies (i.e. 120VAC and 240VAC systems,
or 12V, 24V and 48V DC systems).
Most switching power supply integrated
circuits can be used in multiple topologies.
The manufacturer can provide optional
configuration information of their parts.
Again, in every case, the designer must
evaluate all three portions of the design:
thermal system performance, electrical
system performance, and system assembly
efficiency to properly optimize the system.
Examples of a buck topology switching
current control supply:
Figure 8 shows an example of a buck
topology switcher, the MLX10801 from
Melexis. This circuit features a digital
calibration interface for ‘tuning’ the current
level at production end-of-line, and a remote
temperature sense diode input to shut down
the driver when temperatures reach a
specified maximum. This part is suitable for
automotive applications.
02.02.05 page 9 of 14
Figure 8: MLX10801, an example of a buck topology switcher
Examples of a boost topology switching
power supply:
Figure 9 is an example of a boost topology
switching power supply, the EL7512 from
Intersil.
This circuit features an output over-voltage
protection for applications where the load
may be removed from the supply, and an
extended supply operating range up to 16 V.
The Rset (36K) resistor is used to set the
LED current level, which can have a max
range of 200mA to 500mA depending on the
input supply voltage.
Figure 10 is another example of a boost
topology switcher, the LM2733 from National
Semiconductor. This circuit features
simplicity, and a 1.6MHz operating
frequency for small component size.
Figure 9: EL7512, an example of a boost topology switching power supply
10K
Ω
1.0
Figure 10: LM2733, an example of a boost topology switcher
02.02.05 page 10 of 14
Figure 11 is another example of a boost
topology switching power supply, the
ZXSC400 from Zetex. This circuit features
driving a white Golden Dragon LED from 2
NiMH or NiCd cells, and an external power
switch to allow a wide range of transistors to
be selected based on the current and
voltage needs of the load.
Ω
17 m
Figure 11: ZXSC400, an example of a boost topology switching power supply
Example of a buck-boost topology
switching power supply:
Figure 12 is a concept example of a buckboost topology switching power supply, the
LM2673-ADJ from National Semiconductor.
This circuit can take a varying supply
voltage, and drive a string of LEDs operating
at a voltage in the middle of the range of the
supply voltage.
Ω
17 m
Figure 12: LM2673-ADJ, an example of a buck-boost topology switching power supply
02.02.05 page 11 of 14
Figure 13 is an example of a buck-boost
topology switching power supply, the
HV9906 from Supertex. This circuit is
actually a buck-boost-buck, but it demonstrates the function of a buck-boost. This
circuit offers operation directly from the AC
power mains with power factor correction.
The buck-boost nature allows this circuit to
do the power factor correction. There are no
electrolytic capacitors in the circuit, which
improves long-term reliability. There is a 7V
version of this part allowing it to function in
automotive applications as well.
1N4007
MURS160
65 to 280 V AC
56 µH
MURS160
4.4 uF
15 µH
100 nF
8 M
100 K
Vin
Von
Vdd
AGND
1 µF
Gate
NS
PS
PGND
IRF BC3 0 AF
10 nF
10 nF
MURS160
70
W
Figure 13: HV9906, an example of a buck boost topology switching power supply
02.02.05 page 12 of 14
DIMMING THE GOLDEN DRAGON LED
OSRAM Opto Semiconductors has
published an application note on dimming
InGaN LEDs titled “Dimming InGaN LEDs”.
Since the white Golden Dragon LED is
InGaN based, this application note should
be consulted if the application will be
dimming the LED.
Dimming with a switching power supply can
be difficult. Since the supply is frequency
balanced, rapidly switching the supply on
and off can create instabilities in the system.
Some higher frequency switching power
supply integrated circuits have a dimming
input that allows a PWM input. Others have
an analog input pin that allows dimming by
changing the current level of the LEDs.
(This, as mentioned in the dimming
application note, can change the hue of the
white Golden Dragon LED’s emitted light)
Care must be taken when PWM dimming a
switching power supply, as some parts
accept a PWM input, but convert this input to
a change in the applied forward current of
the LED rather than pulsing the LED and
changing the average forward current.
Consult with the individual integrated circuit
manufacturers on how their parts utilize a
PWM input.
SUMMARY
• To minimize lighting system costs,
and maximize performance, all areas
must be evaluated and optimized:
thermal design, electrical design,
optical design and system assembly
efficiency.
• The Golden Dragon LED can be
driven with a series resistor, but at a
greatly reduced capacity. It needs a
current control supply to maintain
reliable and consistent performance
in most applications.
• The lighting system thermal
performance is key: keep the thermal
resistance of the system as low as
economically possible for best
thermal efficiency to keep the LED
and current control parts count low.
Good thermal management will also
improve the reliability of the system.
• Design the system to keep the LED’s
junction temperature as low as
possible, because a cool LED
generates more light than a hot LED.
In all cases design the system to
prevent the LED’s junction temperature from exceeding the rated
maximums given in the data sheet.
• A linear current control supply is best
suited when the load voltage will be
just below the input supply voltage
(to minimize power dissipation in the
current control supply) and when
there are only a few strings.
• A buck topology current control
supply is used in applications where
the input supply voltage will always
be above the load voltage, and the
power dissipated is more than what
can be reasonably handled with a
linear current control supply.
• A boost topology current control
supply is used when the input supply
voltage will always be below the load
voltage, or when there are many
LEDs to drive. In the later case, with
many strings, several supplies would
be needed, or a current divider would
be needed. Putting the LEDs into
one longer string reduces the total
supply need, but the voltage
increases above the input supply.
This then requires a boost topology
current control supply.
• A buck-boost topology current control
supply is used when the supply and
load voltages will not consistently be
higher or lower than one another. In
some cases, adding LEDs to the
strings and using a boost topology
current control supply can offer a
better solution.
02.02.05 page 13 of 14
APPENDIX
The following is a list of integrated circuit manufacturers and module manufacturers who have a
product that is designed for driving LEDs or can be configured to drive LEDs. This is only
provided as a reference, and is not exhaustive.
Advanced Analogic Technologies Inc. www.analogictech.com
Advanced Transformer Company www.advancedtransformer.com
Allegro Microsystems Inc. www.allegromicro.com
Intersil Corporation www.intersil.com
Fairchild Semiconductor Corporation www.fairchildsemi.com
Infineon Technologies AG www.infineon.com
IXYS Corporation www.ixys.com
LEDdynamics www.leddynamics.com
Linear Technology Corporation www.linear.com
Lumidrives Ltd. www.lumidrives.com
Maxim Integrated Products www.maxim-ic.com
Melexis Microelectronic Systems www.melexis.com
Microsemi Corporation www.microsemi.com
National Semiconductor Corporation www.national.com
On Semiconductor www.onsemi.com
Power Integrations Incorporated www.powerint.com
ST Microelectronics www.st.com
Supertex Incorporated www.supertex.com
Sipex Corporation www.sipex.com
Texas Instruments Incorporated www.ti.com
Toko Incorporated www.toko.com
Zetex Semiconductors www.zetex.com
Author: Timothy Dunn
About Osram Opto Semiconductors
Osram Opto Semiconductors GmbH, Regensburg, is a wholly owned subsidiary of Osram GmbH, one of
the world’s three largest lamp manufacturers, and offers its customers a range of solutions based on
semiconductor technology for lighting, sensor and visualisation applications. The company operates
facilities in Regensburg (Germany), San José (USA) and Penang (Malaysia). Further information is
available at www.osram-os.com.
All information contained in this document has been checked with the greatest care. OSRAM Opto
Semiconductors GmbH can however, not be made liable for any damage that occurs in connection with
the use of these contents.
02.02.05 page 14 of 14
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